| Physiological function can be measured with magnetic resonance imaging (MRI) of different functional contrast. For example, T2*-weighted MRI provides blood oxygenation level dependent (BOLD) contrast, and can detect the hemodynamic responses associated with neuronal activation. MR imaging with BOLD contrast is a useful tool for a noninvasive mapping of brain function, and has a great potential for neuroscience research.; Echo planar imaging (EPI) is one of the fastest MR imaging sequences, and is appropriate for functional MRI (fMRI) studies because of its high temporal resolution and sensitivity to BOLD effect. However, EPI data is always degraded by various artifacts due to off-resonance factors. EPI artifacts need to be removed before the EPI based fMRI data can be correctly interpreted.; In this thesis, three novel techniques have been developed for an effective removal of EPI artifacts. (i) Using the developed two-dimensional phase correction algorithm, the Nyquist ghost artifacts due to EPI odd-even-echo inconsistency can be removed. (ii) Using the proposed phase-shifted EPI sequence and multi-channel modulation post-processing algorithm, EPI geometric distortions due to off-resonance factors and gradient waveform imperfections can be simultaneously corrected. (iii) Using the field map based RF refocusing technique, signal loss due to intravoxel dephasing effect in T2*-weighted images can be effectively recovered. Experimental results from phantom and animal studies demonstrate that the developed artifact removal techniques are more effective than previously reported methods. With the proposed methods, EPI based functional data can be represented in nondistorted coordinates, and correctly compared with anatomic images acquired with other MRI pulse sequences.; The developed EPI techniques have been successfully applied to fMRI studies in conscious rabbits. In studies of sensory system (eye or whisker) activation, reproducible functional activation in the corresponding cortical areas has been observed. In studies of eye-blink conditioning, significant BOLD signal changes in different brain regions, including hippocampus and temporal cortex, have been detected. |
